In the therapy studies ALL-BFM 83 and 86, immunophenotyping of ALL by monoclonal antibodies was performed in a total of 1162 protocol patients (ALL-BFM 83 n = 578; ALL-BFM 86 n = 584). Both studies yielded similar results with respect to the incidence of immunological subtypes: CD10-negative pre-pre-B ALL (ALL-BFM 83: 3.6%; ALL-BFM 86: 5.3%), common ALL (80.1%; 77.9%), B-ALL (1.9%; 2.8%), pre-T/T-ALL (13.9%; 13.5%). Leukemic cells of 3 patients in the ALL-BFM 83 study lacked lymphoid and myeloid antigens (acute unclassifiable leukemia, 0.5%), and 3 patients in the ALL-BFM 86 study exhibited different blast populations with expression of either myeloid or lymphoid features (acute mixed-lineage leukemia, 0.5%). Coexpression of myeloid antigens (CD13 and/or CD33 and/or CDw65) on lymphoblasts (My-positive ALL) was identified in 35 of the 570 (6.1%) protocol patients prospectively analyzed in the ALL-BFM 86 study. The following associations were observed between the immunological subtype and the clinical risk factors: median age (years)-pre-pre-B 3.0, common 4.3, B- 7.9, pre-T/T-ALL 8.5 (pre-pre-B, common vs. pre-T/T-ALL p = 0.05); median leukocyte counts (x 10(9)/l)-pre-pre-B 80, common 9.1, B- 12.3, pre-T/T-ALL 68.1 (common, B- vs. pre-pre-B, pre-T/T-ALL p less than 0.05). The prognostic relevance of the immunophenotype was evaluated on the basis of the therapeutic results obtained in the ALL-BFM 83 study. A significant difference in the remission rate was only recognizable between patients with common ALL (99.1%) and those with pre-T/T-ALL (93.7%, p less than 0.001). After a median follow-up of 54 months, the probability of event-free survival is 71% for pre-pre-B ALL, 67% for common ALL, 56% for pre-T/T-ALL and 27% for B-ALL (common vs. B-, pre-T/T-ALL p less than 0.001), the prognosis in patients with pre-pre-B and common ALL being markedly influenced by the initial leukocyte counts and the age.